Water Structure and Electric Fields at the Interface of Oil Droplets

Mesoscale water-hydrophobic interfaces are of fundamental importance in multiple disciplines, but their molecular properties have remained elusive for decades due to experimental complications and alternate theoretical explanations. Surface-specific spectroscopies, such as vibrational sum-frequency techniques, suffer from either sample preparation issues or the need for complex spectral corrections. Here, we report on a robust "in solution" interface-selective Raman spectroscopy approach using multivariate curve resolution to probe hexadecane in water emulsions. Computationally, we use the recently developed monomer field model for Raman spectroscopy to help interpret the interfacial spectra. Unlike with vibrational sum frequency techniques, our interfacial spectra are readily comparable to the spectra of bulk water, yielding new insights. The combination of experiment and theory show that the interface leads to reduced tetrahedral order and weaker hydrogen bonding, giving rise to a substantial water population with dangling OH at the interface. Additionally, the stretching mode of these free OH experiences a ~80 cm-1 red-shift due to a strong electric field which we attribute to the negative zeta potential that is general to oil droplets. These findings are either opposite to, or absent in, the molecular hydrophobic interface formed by small solutes. Together, water structural disorder and enhanced electrostatics are an emergent feature at the mesoscale interface of oil-water emulsions, with an estimated interfacial electric field of ~35-70 MV/cm that is important for chemical reactivity.